Ok, thanks for answer,
It's really silly to have an official version with GnuTLS if isn't
functionnal :(

GnuTLS is fairly buggy, and also has serious known security issues. GnuTLS
support was added at Debian's request, because of their issues with the
OpenSSL license. That doesn't fix the fact that GnuTLS in and of itself
has issues.

If i change my certificate for gnuTLS generated certificate (i use self
signed certificate..), no need to create a custom OpenLdap with OpenSSL
library ?

It might. I would still suggest you rebuild OpenLDAP so that it is linked
to OpenSSL instead of GnuTLS.